oil very dirty

victory_tea2085June 11, 2013

My 2011 Nissan Sentra has it's oil changed at 3000 miles religiously. I put Mobil synthetic in it. Upon inspection of the oil (once drained) it is very dark and appears dirty. Mechanic told me it was because of the gas having so much ethane. This doesn't sound right to me. What could be the reason the oil is getting so dirty? Thanks- Paul

The oil (and internal engine surfaces) do not get hot enough to drive condensates our of the oil. The stuff hangs around and forms various sludges.

2. The crankcase breather is not working, so blow-by gases and unburned fuel concentrate inside the engine and generally gunks things up.

3. Overheating

If the engine overheats regularily and breaks down oil, expect to see breakdown and oxidized stuff in the oil - black gunk. Towing and long hill climbs like going up to a mountain pass can be overheaters. Also, a defective cooling system.

Late ignition timing can cause overheating. Fuel comsumption is increased to make the same power as before.

4. Worn engine - you did not say how many miles was on this engine.

Worn rings letting excessive blowby to pass into the crankcase.

Leaking exhaust valve seals allowing exhaust gas to pass to the inside of the engine. Improper heat transfer from the exhaust valve stems allowing these to get too hot and ruining the seals.

5. Engine overloading.

6. Scored or non-straight cylinder walls, also worn cylinder walls, or a combination of worn rings and cylinder walls, Run a compression test to check for general piston-cylinder-valve sealing.

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There are some oils that quickly darken with use but are still serviceable.

There's nothing wrong with changing the oil AND FILTER at 5000 miles. On older cars of mine, I rarely go beyond 4000 miles, and more often if cruded up from winter time driving.

Some schedules say change the oil filter every 2nd oil change. I think this is folly - oil filters are not that expensive and do clog up. If the oil needs changing, the filter does too!

If a filter clogs, it presents a pressure drop and its relief valve may open allowing unfiltered oil to go through the system.

By the way, I have no experience with synthetics, but would expect these to darken with time. These are supposed to last longer that regular oils, but in my opinion, any oil no mater its type should be changed when it gets contaminated. Oil quality is sometimes hard to judge when it darkens naturally.

I do have experience with silicone based geases and can not recommend these for hot locations. These lubricants do a fine job unless they are broken down by temperature. At some elevated temperaure, silicon based grease breaks down and one of the products is silicon-dioxide, quartz sand in other words. In one test application I had a slow speed sliding surface that by the nature of the product got very hot (electrical power rehostat). I tried greasing the bearing surfaces with silicon grease, but it soon turned into a grinding compound.